Apparatus and method of compensating a long highly dispersive traveling wave transmission line

ABSTRACT

A method and structure for correcting and linearizing the phase distribution along the waveguide transmission line feed of a long, highly dispersive, slow-wave, N-element antenna array. The method involves the phase probing of the individual radiating elements of a linear array formed by slotting a serpentine waveguide feed. This probing is carried out in predetermined equal increments of Y-elements each. The measured phase over each of the increments of Y-elements is subtracted from a theoretical or reference phase desired at the predetermined element and the phase error ( Delta phi ) thus determined is divided by Y. Individual dielectric panels are affixed to the serpentine broad internal walls (&#39;&#39;&#39;&#39;a&#39;&#39;&#39;&#39; dimension walls). Enough panels are inserted between the radiators of each adjacent pair to provide a - Delta phi /Y phase shift. The process is repeated N/Y times for each linear array. A twodimensional array can be assembled from M of these linear arrays, with each linear array having equal phase distribution characteristics overall and among its N-elements.

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APPAIMATHJSANU METHUW Uh BUMPIEWfiATlING A LUNG HIGHLY lllllfiPESlWlE 'llllAWELlNG WAVE 'llllllhWSMllfiSllUN lLllNE [72] inventors: Earl 1L. Kenworthy,'l.os Angeles', Russell d me pm .m m .m Fl

n .m n N n 3 EU .me d cl! 6 .1. w wv m r be m rg mn c0 mfl a m du mu m mm mm u AW Ill. lllihbs, West Covina, both of Calif.

of a long, highly dispersive, slow-wave N-element antenna ar- [73] Assignee: llnternational Telephone and Telmraph (Iorporallion, New York, lNLY.

ray. The method involves the phase probing of the individual radiating elements of a linear array formed by slotting a ser- [22] Filed:

pentine waveguide feed. This probing is carried out in of Y-elements each.

predetermined equal increments [21] App]. No.: Will 3,105,968 10/1963 Bodmer..............;.................

FORElGN PATENTS OR APPLICATIONS 1,014,722 6/1952 France..................................,343/77l MMWSWW'WWM PAIENTEUFEB 15 I972 SHEET 1 BF 2 L/NEAB AelaAvAzumeees To FAR. F/ELDPO/NT FEEDER NETWORK w, IE w. 6 P m mm 5\/ mm 7 ms 05 0 N M A m E/ m w .m L E md -m Z mmm 4w P6 6 MW CW m i -0 mg 5 0 2 nw Mu FL: w w m w w m a w a w d ELEMENT NUMBER INVENTOR. EARL L. KE'A/WOBTHY EUSSELL E. H1886 PAIENYEUFEB 1 5 m2 SHEET 2 OF 2 YNVENTOR. KE/VWORTHY RUSSELL 1Q. H1555 AGE/VT BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to directive antenna arrays in general and more particularly to means and method for correcting and linearizing the phase distribution among the elements of a long, highly dispersive, slow-wave antenna array.

2. Description of the Prior Art In the prior art, various Radar systems have been devised to make use of the fact that a slow-wave transmission line feeding a linear array provides a structure for inertialess scanning. The transmission line is often folded into a serpentine as shown in U.S. Pat. No. 3,039,097.

Such a configuration is inherently adapted to beam-pointing as a function of frequency of excitation and therefore, inertialess scanning. U.S. Pat. No. 3,438,035 illustrates a system in which a number of serpentine waveguide feed slot arrays are assembled to form a two-dimensional array capable of producing directivity in two coordinates. In this latter prior art patent, the variable frequency excitation produces frequency scanning in one coordinate. The principles describing the operation of such devices as frequency scanners are nowwell known in this art.

A practical problem arises in connection with a scanner constructed with machined waveguide serpentines as depicted in FIGS. I and A of US. Pat. No. 3,438,035. That is, the required machining is extremely expensive if the degree of phase distribution uniformity necessary for development of desired beam patterns in space is to be achieved. Moreover, the beam-pointing angle for each linear array as a function of frequency of excitation must be the same within a close tolerance. The present invention addresses itself to the aforementioned problems.

SUMMARY In view of the practical problems presenting themselves in prior art structures of the character described, it may be said that the general object of the present invention was the development of a method and structure for linearizing and making uniform the overall phase distribution of a linear waveguide-fed slow-wave array. A number of such arrays can then properly be assembled into a two-dimensional array having predictable pattern and scanning characteristics. As a corollary to this object, it might be said that it was intended to accomplish the objective with machined (or otherwise fabricated serpentine) waveguide structures built to reasonable manufacturing tolerances.

The showing of U.S. Pat. No. 3,438,035 presumes that the serpentines are machined in halves and assembled mechanically. This particular structure is especially adaptable for use of the present invention, as will be realized as the description proceeds.

The basis of the present invention is the observation that partial loading of rectangular waveguide with a dielectric slab or panel, perpendicular to the lE-field, will delay the electromagnetic wave proceeding along the guide. For a dielectric slab or panel extending along the entire broad dimension (a dimension) as an air-filled waveguide, it can be shown that where:

a, guide wavelength in partially filled guide A free space wavelength d thickness of dielectric panel b narrow dimension of rectangular waveguide a wide dimension of rectangular waveguide e, relative dielectric constant of dielectric panel Experimental investigation shows that the guide wavelength above described is shorter than the guide wavelength in a cor responding piece of waveguide minus the dielectric panel. Therefore, for equal lengths of identical waveguide, one with dielectric panel and one without, the one with the panel will delay an electromagnetic wave more or cause it to experience more negative phase shift in travel through the length. The socalled dielectric panels of the present invention are actually strips of a dielectric tape selected for its low loss and ease with which it can be installed. The above-described basic principle is that by which the tape strips of the present invention work, i.e., they cause greater delay in the field than a corresponding section of the slow-wave structure not having the strips. it is the ability of the tape strips to perturb the propagation constant that produces the effect. The effect of the tape strips upon the propagation constant can be inferred from the above formula, however, a carefully controlled experimental program has been shown to be a reliable empirical way of ascertaining with accuracy the effect of a given tape strip on the propagation constant.

The method involved in applying the dielectric tapes involves first the determination of phase shift experienced by the electric field.

The machined-in-halves serpentine structure referred to is obviously readily adapted to the installation of the tape strips to perform the dielectric panel function, however, as a first step, the serpentine is assembled without the tapes and. is phase-probed. 1

A particular application of the present invention was undertaken in a serpentine built to operate in the vicinity of 9.0 Gl-lz. Phase probing was thus readily accomplished using wellknown microwave test equipment.

In phase probing, the phase of energy at predetermined equal increments (of Y-elements each) along the array, is determined with respect to the input of the waveguide. Thus discrete corresponding points on a phase distribution curve are obtained. The required correction (Ad) to linearize the curve at these points is then readily determined and converted to a number of discrete dielectric unit strips each of small predetermined phase shifting equivalence. These are then uniformly distributed on the a dimension serpentine interior walls among the paths separating adjacent radiators; -A/( phase shift thereby being introduced between adjacent radiators.

Further detail will be added in description taken against the drawings hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic of a two dimensional array assembly of M-linear arrays in which the present invention is useful.

FIG. 2 is a schematic of one N-element slotted linear array of the FIG. I assembly of Mlinear arrays.

FIG. 3 is a phase distribution graph to illustrate correction of two typical serpentines according to the present invention.

FIG. A is an isometric partial view of the two halves of a serpentine waveguide structure prior to assembly but having dielectric panels emplaced according to the present invention. FIG. 5 is a partial lateral view of half of the structure of FIG.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. l, the two dimensional array depicted schematically could well be that shown in U .5. Pat. No. 3,438,035. This showing is for the purpose of establishing element and linear array identification and relationship, N" and M, respectively, for purposes of the description. The feeder network I could readily be the variable waveguide-width phase distributor assembly of that patent, and the typical slot radiators 2 and 3 in the Mth linear array in FIG. I similarly could be corresponding slots. The line termination or serpentine load A is a straight forward expedient, well understood in the art and could be provided by any of various well-known alternatives.

In FIG. 2, the serpentine shown schematically could represent that of any of the linear arrays 1 through M of FIG. 1, the slot element numbers ll through N also corresponding. The distance d represents the external or array surface spacing of elements as contrasted to S, which represents the path length between radiators within the serpentine waveguide. The array normal is represented by a vector 7 and the wave front line 5 is that which would correspond to an arbitrary frequency of excitation producing a corresponding phase distribution over the 1 to N elements.

The vector 6 then points in the direction of radiation (i.e., the normal to the phase field line 5) this vector 6 makes an angle 6 with the array physical normal 7.

FIG. 3 depicts the uncompensated phase distributions of two arbitrary selected finish machining serpentines A and B at curves 6 and 7 respectively. Curve 5 is a theoretical or reference phase line constituting the ideal to which A and B serpentines are to be corrected in accordance with the method and structure of the present invention.

Before proceeding with description of the remaining figures, some additional theoretical discussion of the problem and its solution is in order.

In order to focus the array beam at some distant point in space, it is highly desirable that the equiphase contours of the radiated field lie in planes. In order to accomplish these planar phase contours in the far field, it is necessary, as already indicated, that the phase distribution along each of the linear arrays be itself linear. Of course, the phase distribution across the feeder network must also be linear. It is not sufficient that each linear array's distribution have the same slope; the phase distributions along each linear array must, in fact, be equal within a very close tolerance. The so-called serpentine fed linear array with which the present invention is primarily concerned, is broadly classed as a traveling wave array, such arrays being frequency scannable over an angle 9, to 9 as the frequency is changed from j to f In most applications, it is desirable to scan the beam over a relatively wide range of angles using only a moderate frequency range. Ordinarily this dispersion is accomplished by utilizing a slow-wave structure, the geometry of which is depicted in FIG. 2.

Although the present invention is being described as though the individual radiators in the broadwall of the serpentine waveguide are slots, it is, nevertheless, possible for them to be individual dipoles or some other type of radiating element.

In a serpentine waveguide per se, the waveguide is alternately folded back on itself as shown in the referred prior art, in order that the mechanical distance between radiators measured within the waveguide is substantially greater than the straight line distance between radiators along the array surface.

In practice, the distance d from FIG. 2 will be somewhat less than a free space wavelength, while the distance S may be many multiples of a half wavelength within the guide. The result is a highly dispersive structure, that is, one in which relative phase between adjacent radiator changes very rapidly as the frequency of excitation varies.

For a typical practical planar array, operating in the 9.0 GI-Iz. region, M" and N may well both be numbers in excess of 150. In manufacturing, a serpentine slowwave structure of this type can be produced by clip brazing and assembly of alternate straight pieces of waveguide to a series of 180 bends. This manufacturing technique is generally thought to be suitable where the resulting bulk is not a limiting disadvantage and the tolerances and repeatable requirements are not stringent, for instance, such a fabrication would be more applicable where only one serpentine structure was required, as for example, in 11.5. Pat. No. 3,039,097.

In instances where many highly compact identical slowwave structures are required for the construction of a two dimensional planar array, the total machining technique is more applicable. In this technique the individual slow-wave structures are machined out of solid blocks of material, as for example, through the use of a digitally programmable'end milling machine. Each serpentine is milled one-half at a time,

The more critical machining dimension in the totallymachined serpentine structure is the depth of cut into the two halves as this dimension controls the width of the waveguide broadwall (a dimension) and consequently determines the phase constant of the waveguide. To examine this phase constant relationship in a rectangular waveguide, which the preferred serpentine structure actually is, in extended and folded form, the following may be written.

B phase constant in radians per unit length k guide wavelength A free space wavelength a waveguide broadwall dimension Differentiating with respect to a yields:

6B 1r A a3 Wl'llCl'l approximately equals A In the following, the inch is the unit of measurement, and

AB phase change per inch,

ABS phase change in degrees between elements, and AB-S'N phase change in degrees across the N-element structure.

The problem of repeatability and accuracy being the primary manufacturing problem, the development of a unique method of manufacturing M substantially identical N-element structures insofar as phase distributions are concerned, was one of the most pressing in practical instrumentation of slowwave structure.

To illustrate further, assume the following parameters:

A very liberal criterion would be a requirement that the maximum difference in phase between the ends of any two of the M slow-wave structures be less than If this value is substituted into the expression AB'S'N, one finds that Aa= 0.005 inch. In effect, this imposes a tolerance on the a dimension of the waveguide of 10.00025 inch. Within the present state of the art, this tolerance is not at all practical, in that its attainment would make the manufacturing process extremely costly.

In respect to the difficulty of the foregoing, the placement of dielectric panels perpendicular to the E-field within the waveguide (serpentine) to effect partial loading, and consequently to introduce phase delay, was conceived.

The phase probing of each slow-wave serpentine structure in order to obtain the data depicted in the A and B curves of FIG. 3 can be carried on with sufficient accuracy well within the capabilities of microwave test instruments (such as RF- phase bridge) currently available, and whereas the required total'machining tolerances were obviously difficult to achieve even with the more advanced machining methods.

A program of experimentation designed to select a practical material for the aforementioned dielectric panels was conducted. Capabilities of various dielectric materials for producl0l023 ()llll ing the desired phase shift were examined in a test waveguide. Tape, identified commercially as X-774-l dielectric tape, manufactured by the Schjeldahl Co., was selected. Generically, this tape is a multilayer-type comprising a Tedlar" layer as an outside coating supported by a Dacron" cloth layer and a layer of Mylar." A good pressure-sensitive adhesive surface on this tape contributed also to its selection. The actual choice of material for the dielectric panels is open to selection among a substantial variety of dielectric materials available. The criteria are not unlike those relating to selection of capacitor dielectric layers. This is to say, reasonably good dielectric strength and low-loss characteristics, as well as phase-shifting ability in the application, at the microwave frequency involved are among the most significant considerations.

Referring now to H6. 4, the emplacement of the tape strips, constituting the dielectric panels as aforesaid, is illustrated. Two halves of serpentine 8 and 9 are illustrated with the dielectric tapes installed and ready for assembly. For clarity, the showing is of only a partial serpentine length since, as previously indicated, the number of radiating slots in a practical X-band array might well be on the order of 150 or more.

The radiating slots lull and ill are assumed, for the sake of example, to be the N-3 and N- l elements. The dielectric panels across the broadwall of the completely assembled structure are then actually two in number, such as for example, l7 and companion illa, when the final assembly of 8 and 9 is effected. The other panel pairs called out are then 11d and lba, l9 and Wu, and also 20 and 20a. The inside webs which form the said broadwall, such as l2, 13, M, 115 and i6 (typically) are butted against 12a, 13a, Ma, 15a and 16a, as will be apparent from FIG. 4.

Referring also to FIG. 5, a lateral view looking into 9 is presented, showing dielectric panels ll, 22 and 23 not visible in H6. 4. These panels would normally also have companion panels not visible. The use of these parallel panel pairs makes the broadwall loading symmetrical and is desirable as far as possible as an expedient for preventing the appearance of higher order moding in the serpentine.

From an understanding of the dielectric panel installations, it follows from H6. 5 that the path S between adjacent slot radiators l0 and ii can include up to eight panels of the relative size illustrated, i.e., 22, it 23 and 119 in serpentine half 9 and their companion panels in 8.

Of course, in a serpentine requiring relatively little correction, only one or two broadwall pair of panels may be required between slots, rather than the indicated four pairs. Each panel contributes its increment of phase delay, whether in series or parallel with another panel within the guide.

The typical drilled web boss 2d accommodates the bolting together of h and 9. The shape of the internal web boss, such as at 25, is not a part of the present invention, but is to be regarded as one of various internal configurations possible in the making of a machined serpentine. The dovetail internal construction illustrated in US. Pat. No. 3,438,035 is one of these possible variations.

Concerning the practical procedure for sizing the tape strips to function as dielectric panels, it was experimentally determined that a suitable tape strip for use in an X-band serpentine of the type illustrated was 1 inch long and 0.375 inch in width. The effective incremental phase shift per strip was 0.95 at 9.0 GHZ. In one practical embodiment, the correction required over the first elements was determined by phaseprobing at element number 20 to be degrees. filecordinw 147 tapes were required to be distributed uniformly between the first and twentieth element.

It will be understood that other variations are'possible in the precise method and structure of the present invention, and these will be apparent to those skilled in this art, once the present invention is understood.

The invention is, of course, capable of implementation at frequencies other than the X-band assumed in the description.

What is claimed is: i l. The method of achieving linear phase distribution over the length of a waveguide transmission line of the slow-wave type, comprising:

measuring the phase of radiofrequency energy at a first predetermined point located at a distance D from the energized end of said transmission line with respect to the phase at said energized end to determine a first uncorrected cumulative phase; subtracting said first uncorrected phase from a first predetermined theoretical cumulative phase at said first point to determine a first phase error, Adz;

applying dielectric panels N in number and each capable of producing AdJ/N phase delay, uniformly distributed along the inside wall of said waveguide substantially perpen' dicular to the electric field therein; and repeating said process steps discretely for each successive increment equal to said D along said line.

2. A linear antenna array which includes a plurality of radiating elements fed from successive points along an ex tended path rectangular waveguide transmission line for developing an electromagnetic beam narrow in at least one polar scan coordinate, said beam being generated at a pointing angle which is a function of frequency of the excitation energy within said waveguide, comprising:

means comprising at least a partly folded structure for said waveguide to provide said extended path for obtaining a predetermined rate of change relationship between said pointing angle and said frequency of excitation;

and means for linearizing the overall phase distribution along said feed points comprising a plurality of dielectric panels uniformly placed at intervals along the length of said folded waveguide, at least one of said panels being located against the a dimension interval wall of said waveguide.

3. Apparatus according to claim 2 in which said folded waveguide structure is defined as a serpentine-shaped array feedline.

4. The invention set forth in claim 3, further defined in that said waveguide is fabricated in two parts, said two parts being such as would be formed by cutting said serpentine lengthwise bisecting said a dimension internal walls, and said dielectric panels are mounted substantially symmetrically on said a dimension wall on both sides of said bisection, thereby to afford installation of said dielectric panels before said serpentine waveguide is fully assembled.

3. The invention set forth in claim 3, further defined in that said dielectric panels are individually located between adjacent radiating element feed points along substantially uncurved walls of said serpentine waveguide.

ti. The invention set forth in claim 3, further defined in that said dielectric panels are individually distributed along all substantially uncurved a dimension intemal walls of said waveguide serpentine.

i t l i i 

1. The method of achieving linear phase distribution over the length of a waveguide transmission line of the slow-wave type, comprising: measuring the phase of radiofrequency energy at a first predetermined point located at a distance D from the energized end of said transmission line with respect to the phase at said energized end to determine a first uncorrected cumulative phase; subtracting said first uncorrected phase from a first predetermined theoretical cumulative phase at said first point to determine a first phase error, Delta phi ; applying dielectric panels N in number and each capable of producing Delta phi /N phase delay, uniformly distributed along the inside wall of said waveguide substantially perpendicular to the electric field therein; and repeating said process steps discretely for each successive increment equal to said D along said line.
 2. A linear antenna array which includes a plurality of radiating elements fed from successive points along an extended path rectangular waveguide transmission line for developing an electromagnetic beam narrow in at least one polar scan coordinate, said beam being generated at a pointing angle which is a function of frequency of the excitation energy within said waveguide, comprising: means comprising at least a partly folded structure for said waveguide to provide said extended path for obtaining a predetermined rate of change relationship between said pointing angle and said frequency of excitation; and means for linearizing the overall phase distribution along said feed points comprising a plurality of dielectric panels uniformly placed at intervals along the length of said folded waveguide, at least one of said panels being located against the a dimension interval wall of said waveguide.
 3. Apparatus according to claim 2 in which said folded waveguide structure is defined as a serpentine-shaped array feedline.
 4. The invention set forth in claim 3, further defined in that said waveguide is fabricated in two parts, said two parts being such as would be formed by cutting said serpentine lengthwise bisecting said a dimension internal walls, and said dielectric panels are mounted substantially symmetrically on said a dimension wall on both sides of said bisection, thereby to afford installation of said dielectric panels before said serpentine waveguide is fully assembled.
 5. The invention set forth in claim 3, further defined in that said dielectric panels are individually located between adjacent radiating element feed points along substantially uncurved walls of said serpentine waveguide.
 6. The invention set forth in claim 3, further defined in that said dielectric panels are individually distributed along all substantially uncurved a dimension internal walls of said waveguide serpentine. 